Metal Oxide Semiconductor Field Effect Transistor (MOSFET) technology is well-known, having been invented during the 1950s. Since 1970, it has become a standard technology for producing integrated circuits (ICs) in the semiconductor industry because of its ease of use, low current consumption and low production costs.
In its simplest implementation, a MOSFET is a three-terminal switch, typically fabricated on silicon substrate, having an insulated control gate terminal over a drain-to-source conduction channel. The current flowing in the conduction channel is typically controlled by a applying a voltage to the gate terminal. Being simple switches, MOSFET's are well suited to logic operation. They are also a good choice for low power applications as they consume very little current in operation, primarily because the control gate is insulated from the conduction channel.
MOSFET's can be fabricated to operate in two fundamentally different ways, commonly called enhancement-mode and depletion-mode, operation. In enhancement-mode operation, the MOSFET is in an “off” state unless a voltage is applied to the gate terminal to switch the transistor to an “on” state. In contrast, in depletion-mode operation, the MOSFET is in an “on” state and requires a voltage applied to the gate terminal to switch the transistor to an “off” state.
MOSFETs operating in enhancement-mode are often termed enhancement type MOSFETs and may be either N-channel or P-channel. Similarly, MOSFETs operating in depletion-mode are often termed depletion type MOSFETs and may be either N-channel or P-channel.
Because of there opposite default, or initial state, the logic operation of enhancement type MOSFETs is the opposite of the logic operation of depletion type MOSFET. Other than the difference of polarity, however, both types of MOSFET are theoretically identical in performing all logic operations.
The transfer characteristics of MOSFETs are shown in FIG. 1, with the voltage between the gate terminal and the source terminal (Vgs) plotted against the current flowing from drain to source (Ids).
The enhancement type MOSFET is simple to use, since the channel between drain and source becomes conductive only after the gate to source junction is energized, as shown in curves 142 and 144. The N type enhancement MOSFET is initially off, with no current Ids flowing when Vgs is zero, and becomes more conductive as VGS is made more positive, allowing a greater current Ids to flow from drain to source, as seen in curve 142. Similarly, the P type enhancement MOSFET is initially off, with no current flowing when Vgs is zero, and becomes more conductive, allowing a greater current to flow from drain to source as VGS become more negative, as seen in curve 144.
In contrast, a depletion type MOSFET can be thought of as having two modes of operation, as shown in curves 114 and 116. For instance, the N-type depletion MOSFET 114 has a depletion-mode in which the bias voltage at the gate, Vgs, is either zero or negative. In this mode, the N-type depletion MOSFET is “on”, allowing a current to flow from drain to source, when the voltage is zero. As the bias voltage at the gate, Vgs, is made more negative, the current decreases and eventually stops, so that the MOSFET is “off”.
The other mode of operation of the N-type depletion MOSFET is the enhancement mode, when the bias voltage at the gate, Vgs, varies from zero to more positive. At zero voltage, a current flows that may be considered as a large leakage current. As the bias voltage at the gate, Vgs, is increased the drain to source current increases, or is enhanced, just as in an N-type enhancement MOSFET.
The depletion type MOSFET operation, shown in curves 114 and 116, may also be thought of as a shifted version of enhancement type MOSFETs. For instance, N type depletion MOSFET curve 114 is similar to the N type enhancement MOSFET of curve 142, with a shift in the bias voltage at the gate, Vgs. This simplistic view allows the same model to be used to simulate both the enhancement type MOSFET and depletion type MOSFET, with appropriate change of the potential of the transistor.
This simplistic way of treating a depletion type MOSFET as a gate-bias shifted enhancement type MOSFET has a disadvantage. The simple treatment obscures the fact that the two types of MOSFET may be used to implement opposite logic, i.e., enhancement MOSFETs are “off” by default and may be turned “on” by an appropriate input, while depletion MOSFETs may be operated as “on” by default switch that may be turned off by an appropriate input.
Unfortunately, this simplistic treatment of depletion MOSFETs appears to have been built into the Simulation Program with IC Emphasis (SPICE). SPICE is an important software tool, originally developed by Nagel and Pederson at the University of Berkeley and released into the public domain in 1972. Since then, SPICE has become widely used in the semiconductor industry for designing integrated circuits.
Attempts to model depletion-mode MOSFET logic circuits in SPICE apparently result in what appears to be a pin assignment error or bug. This bug is further obscured by the fact that SPICE, apparently, does a good job of simulating the enhancement-mode behavior of a depletion type MOSFET.
The net effect of the simplistic model used for depletion-type MOSFETs appears to have been the neglect of depletion-mode logic circuits in IC design at the expense of enhancement-mode logic. This is a major oversight, as enhancement-mode MOSFET logic can only be used for “negative-logic” circuits such as, for instance, Boolean NAND or NOR gates, rather than “positive-logic” circuits such as, for instance, AND or OR gates.
What is needed is a method to deal with the SPICE pin assignment bug so that the software can be used to accurately predict the depletion-mode, or “positive-logic”, behavior of depletion type MOSFETs. Such a method will allow the simulation of many important and novel depletion-mode circuits, confirming them as novel, workable IC designs.